Natural killer (NK) cells represent a small population of cytotoxic cells in normal peripheral blood that appear morphologically to be a homogeneous population of large granular lymphocytes. Despite their uniform morphology, NK cells have been attributed to have a variety of diverse immunologic functions including resistance to malignant or virally transformed cells, rejection of allogeneic grafts and hematopoietic regulation, and phenotypic studies have indicated that they express a variety of both T cell and myeloid surface antigens. In order to analyze the structural basis by which NK cells interact with target cells in the context of this extensive heterogeneity, we have developed methods for initiating and maintainting growth of NK cells in vitro. This has resulted in the establishment of a large series of clonal cell lines which maintain NK activity after prolonged periods of culture. Both phenotypic and functional analysis of these NK clones suggests that they accurately reflect the diversity of NK cells in peripheral blood. Based on the phenotype of these NK clones, three major groups have been identified: 1) cells that express no lineage associated antigens; 2) cells with a mautre T cell phenotype (T3+T11+); and 3) those with various combinations of myeloid and T cell antigens, but without expression of T3 antigen (T3-T11+). Recent experiments with T3+T11+ NK clones have demonstrated that even though they do not have MHC restricted CTL-like specificity, these cells utilize a functional T cell receptor to specifically interact with a target antigen that is expressed on various hematopoietic cells after cell activation. Although the analysis of T3+T11+ NK clones has identified a unique functional population of cytolytic cells, other types of NK clones do not express clonotypic T cell receptor-like structures and must therefore utilize other mechanisms for interaction with target cell antigens. In future studies we propose to continue the analysis of NK cell function utilizing monoclonal antibodies to identify functional structures on NK clones. In addition to further studies with T3+, T11+ clones, major efforts will be directed towards the characterization of functional antigens on T3-T11+ NK clones that represent the majority of NK cells. Utilizing monoclonal antibodies directed at NK associated antigens as well as antibodies to functional NK structures, we will also be able to extend our analysis to unstimulated NK cells in peripheral blood. Finally, NK clones will also be utilized to analyze potential mechanisms for the regulation of NK cell activity.